Analytes are traditionally recovered from liquids and solid matrices using solvent extraction methods. By 1995, government-funded contract and selected Federal laboratories will be required by the Environmental Protection Agency (EPA) to reduce their uses of specified solvents by 50%. Supercritical fluid extraction (SFE) offers several advantages as a replacement for solvent-based techniques for analyte isolation and may aid laboratories in complying with such EPA regulations. Supercritical fluid's (SF's) advantageous properties are due to their behavior as solvents above their critical temperatures and pressures. Above the critical point, SFs exhibit properties which are more liquid-like than gaseous. Unlike liquids, SFs are highly compressible above the critical temperature and the density of the fluid increases with small changes in pressure. Although the density of an SF increases with pressure, the transport properties of the fluid remain between that of a gas and a liquid. Supercritical fluids have high diffusivities and low viscosities which may result in a rapid mass transfer of a solute from a sample matrix compared to a liquid. To be usable for analytical supercritical fluid extraction (SFE), a fluid must have certain critical properties such as a low critical temperature and pressure that place it within the operational range of available instrumentation. One fluid that has such properties is carbon dioxide, which has a critical temperature of 31.degree. C. and a critical pressure of 1071 psi (74-bar). Additionally, carbon dioxide is non-toxic, available in high purity and disperses as a gas after depressurization. Because of its desirable properties carbon dioxide with or without modifiers is a preferred fluid for most SFE applications.
Conceptually, the design of a SFE apparatus is relatively simple. However, to fully exploit the potential properties of a SF, the design of each component of the apparatus must be optimized. In most SFEs, the fluid is compressed above its critical pressure using some type of high pressure pump. The compressed fluid then is passed through high pressure tubing into a temperature controlled environment above the critical temperature and through a heat exchanger prior to passage into an extraction vessel. The fluid, above the critical temperature and pressure, flows into the extraction vessel filled with the sample matrix. The SF, laden with solute, passes from the extraction vessel into a restrictor in which the SF is decompressed or depressurized. The restrictor is interfaced with a collector for liquid, open tube, or sorbent trapping. The decompressed gas is swept through the collector in which the sorbents are retained. The possibility of analyte loss at the restrictor/collector interface for most SFE designs is significant.
In general, most SFEs which are laboratory assembled or available commercially use one of two basic types of restrictor designs, fixed or variable flow. A fixed flow restrictor maintains a specific, constant flow rate for each pressure level dependent upon the inside diameter of the tubing used. Fixed restrictors are made from either fused silica capillary or stainless steel tubing. The internal diameters of these restrictors range from 5-55 .mu.m. Because these restrictors are fabricated from narrow bore tubing, they are prone to various problems during operation. A pressure differential may occur along the length of the tubing of the fixed restrictor resulting in the deposition of polar analytes; plugging due to particulate carry over from the extraction vessel; solute deposition at the restrictor entrance; and blockage at the restrictor exit due to low temperature freezing of decompressed gases. The later phenomenon is especially apparent when biological samples, high in fat content, are extracted using SFE equipment with a fixed restrictor. The fat exiting the restrictor solidifies at the exit port causing blockage of flow. Fixed restrictors used in commercial SFEs normally are designed to direct the stream of depressurizing gas laden with solute into test tubes or vials where the analytes are trapped. The collection vessel may contain an organic solvent to assist in retaining the analyte. Current manufacturers whose instruments embody this design are Dionex of Sunnyvale, Calif., Isco of Lincoln, Nebraska, and Suprex of Pittsburgh, Pa.
Several types of variable flow restrictors have been proposed for use as pressure and flow regulators for supercritical fluid extractors. An automatic pulsed flow metering valve for SFE was reported by Saito et al. (Chromatographia 25,801, 1988). The unit has been commercialized by Jasco, Inc. of Easton, Md. This device operates by discharging the analyte into a collection tube or an optional fraction collector. A similar type of pulsed restrictor is offered by Hewlett-Packard of King of Prussia, Pa. on its SFE. The gas stream from the restrictor flows through a permanent solid phase extraction cartridge where analytes are collected. Analytes are recovered from the sorbent in this cartridge by pumping solvent through the cartridge. The solvent laden with analyte is deposited in vials located in an automatic sampler device.
Non-automated variable-flow restrictors are marketed by two companies as part of their SFEs. The Suprex Co. of Pittsburgh, Pa. uses a Variflow restrictor which consists of an elastic tube compressed by an elastic ferrule in a high pressure union. A variable restrictor distributed by CCS Corp. of Avondale, Pa. uses a similar concept in its design. Both devices may be difficult to heat uniformly. Additionally, interfacing such devices to accept standard commercial solid phase extraction (SPE) collectors may be difficult.
Micrometering needle valves have been used with considerable success as variable flow restrictors in supercritical fluid extraction applications. Such valves have a low coefficient of flow (Cv) to meter supercritical fluids at levels needed for analytical applications. To be useful in SFE apparatus, it is desirable that the metering valve be uniformly heated.
There are several manufacturers of micrometering valves which are suitable for use in SFE apparatus. Micrometering valves from Autoclave Engineering, Inc. of Erie, Pa., High Pressure Equipment Company of Erie, Pa. and Butech of Erie, Pa. have been tested and have performed satisfactorily.
As indicated, various types of solid-phase extraction collectors or columns are used with SFE apparatus. However, where routine sampling of products, as well as laboratory sampling is done, it is desirable, more efficient, and more convenient to make use of standard commercially available units. It is also preferable to standardize on the collectors used. Standard collectors come in several sizes, including 3 mL and 6 mL. The smaller unit has an inside diameter of 0.352 inches, a body length of 2.5 inches and an overall length of 2.94 inches. The larger unit has an inside diameter of 0.50 inch, a body length of 2.625 inches, and an overall length of 3.076 inches. Sorbent packing in such collectors has a nominal depth between 0.5 inch to 1.0 inch.
Notwithstanding, the availability of SFE apparatus and commercially available collectors, the accuracy and consistency of results utilizing such equipment is questionable. One of the factors contributing to problems with SFE apparatus and associated collectors may be the path length between the restrictors and SPE collectors and the means of their attachment to such restrictor. Extended length of the tubing pathway from a restrictor to a collector sorbent bed results in analyte deposition in the tubing rather than on the sorbent, a problem of critical importance when analytes are recovered in the ppm to ppb concentration range.